Arrangement for recognising the type of occupation of a vehicle seat

ABSTRACT

An arrangement, having at least one motion sensor in the vehicle seat, detects with a high reliability whether or not the vehicle seat is occupied by a person. For this purpose, the sensor output signal is analyzed into multiple signal components by frequency selection, and decision-making criterion for occupancy or non-occupancy of the vehicle seat by a passenger is formed by comparing the frequency-selective signal components.

BACKGROUND INFORMATION

The present invention concerns an arrangement: for detecting the type ofoccupancy of a seat in a motor vehicle.

It is known that airbags are provided in motor vehicles not only for thedriver but also for the passenger. However, the passenger's airbagshould be activated in an accident only when the passenger's seat isactually occupied by a passenger. If the passenger's seat is notoccupied or is occupied only by baggage or a child's seat, activation ofthe airbag should be suppressed. Activation of an airbag in such caseswould cause unnecessary repair costs and could actually be harmful forother occupants of the vehicle due to the additional increase inpressure generated in the vehicle interior. European Patent 0 458 102 A1proposes deriving a force and/or pressure distribution pattern by meansof force and/or pressure sensors to obtain detailed informationregarding whether the passenger's seat is occupied by a person orbaggage. An example of a force and/or pressure sensor is thepressure-dependent resistance mat (interlink mat) is described in GermanPatent DE 42 37 072 C1. In Patent Abstracts of Japan, vol. 018, no. 620(M-1711), Nov. 25, 1994, JP 62 39 173, an arrangement for detecting seatoccupancy is described. In this arrangement, several filters areprovided to analyze by frequency selection the output signal of at leastone sensor provided in the vehicle. A decision-making criterion forwhether or not the vehicle seat is occupied by a passenger is obtainedby comparing the frequency-selective components.

SUMMARY OF THE INVENTION

The object of the present invention is to create an arrangement fordetecting the type of occupancy of a seat in a motor vehicle. That willgenerate the most reliable possible criterion for deciding whether ornot a vehicle seat is occupied by a person.

The present invention is based on the finding that a vehicle seatoccupied by a person is exposed to different dynamic movements than whena rigid object such as baggage or a child's seat is placed on it. Aperson's inherent motion and even his pulse will induce vibrations ofdifferent frequency ranges than a rigid object in the seat and a motionsensor provided in the seat. If the output signal of the sensor isanalyzed, different frequency ranges are dominant with a person sittingon the seat than with a rigid object. Frequency-dependent analysis ofthe sensor output signal thus leads to a very reliable conclusionregarding whether the vehicle seat is occupied by a person or a rigidobject. Since vehicle operation parameters--e.g., engine rpm or vehiclespeed--also have an effect on the occupancy sensor in the vehicle seat,it is expedient to include these parameters in the decision-makingcriterion derived from the sensor output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates an exemplary embodiment of the arrangementaccording to the present invention.

DETAILED DESCRIPTION

The single FIGURE shows a block diagram of an arrangement that detectswhether or not a vehicle seat is occupied by a person. Only if a personis sitting on the vehicle seat, i.e., the seat is neither unoccupied noroccupied by a rigid object (e.g., baggage or a child's seat), should arestraint system (airbag and/or seatbelt) be activated in an accident,for example, or a seat heating system be activated. For this purpose,the vehicle seat is equipped with one or more sensors 1. The sensormight be, for example, a piezoelectric cable which is installed in theseat and undergoes changes in its conduction properties as a function ofthe vibration to which the vehicle seat is exposed because of a personsitting on it or an object placed on it. Instead of a piezoelectriccable, sensors operating according to other physical principles may alsobe used, e.g., pressure-dependent resistance maL, capacitive sensors,optical fibers, etc., which react similarly to vibrations of the vehicleseat.

The output signal of sensor 1 is sent to an amplifier 2. To this isconnected a filter bank 3 which has the function of selecting the sensoroutput signal according to frequency. Multiple signal components of thesensor output signal, each with its own frequency range, are thusapplied to the outputs of filter bank 3. Filter bank 3, decomposing thesensor output signal into individual frequency ranges, may comprisedigital or analog low-pass or high-pass or band-pass or band-stopfilters. The frequency-selective signal components of filter bank 3 aresent to a selector module 4. Of all the frequencyselective signalcomponents of filter bank 3, the selector module selects two. A circuitblock 5 either forms the mean value or the effective value for each ofthe two selected frequency-selective signal components or determines thepeak value, the mean value or any other signal values suitable forfurther signal processing. The values of the two selectedfrequency-selective signal components appearing at the outputs ofcircuit block 5 are compared in a downstream circuit block 6. This meansthat either the quotient of two signal components or their difference isformed. Output signal X of circuit block 6 then shows, for example,whether the signal component of the lower frequency range was larger orsmaller than the second signal component of the higher frequency range.It has been found experimentally that if the vehicle seat is occupied bya person, the lower frequency components in the sensor output signal arepredominant over the higher frequency components, and with other objectsthe higher frequency signal components are predominant over the lowerfrequency components. For the case when the lower frequency component ispredominant and the low frequency signal component appears in thenumerator when a quotient is formed in circuit block 6, output signal Xof circuit block 6 assumes a value greater than 1. If a comparison ofdifferences is performed between two signal components, output signal Xof circuit block 6 is positive when the lower frequency signal componentis the subtrahend and the higher frequency signal component is theminuend. Signal X goes to a threshold value decision circuit 7 havingtwo thresholds S1 and S2, S1 being larger than S2. If signal X isgreater than threshold S1, threshold value decision circuit 7 willsignal with its signal X1 that the vehicle seat is occupied by a person.However, if signal X is less than lower threshold S2, the thresholdvalue decision circuit will deliver signal X2, meaning that the vehicleseat is not occupied or is occupied by another object. If signal X fallsin the range between the two thresholds S1 and S2, the threshold valuedecision circuit will generate a signal X3. The range between the twothreshold values S1 and S2 is an uncertainty range permitting noaccurate inference regarding the occupancy of the vehicle seat. It isthen appropriate to perform another comparison between two otherfrequency-selective signal components from the filter bank. Outputsignal X3 of threshold value decision circuit 7 signals to selectormodule 4 that such a condition exists. If a new comparison of twofrequency-selective signal components does not lead to a definite resultX1 or X2, another comparison procedure is performed, possibly with thethreshold values being different in the individual comparisonprocedures.

Using two thresholds S1 an S2 in threshold value decision circuit 7greatly reduces the probability of false decisions. However, if littleor no tolerance is demand in the decision about whether or not thevehicle seat is occupied by a person, a threshold value decision circuit7 is provided with only a single threshold.

To perform additional comparisons between frequency-selective signalcomponents in chronological succession instead of as described in thepresent embodiment, selector module 4 may also select multiple pairs offrequency-selective signal components of filter block 3, and multiplepairs of frequency selective signal components may also be comparedsimultaneously and sent to a threshold decision circuit. Such athreshold decision circuit, which would need to provide only onethreshold for each signal component pair, would make a decision for eachsignal component pair regarding whether the vehicle seat is occupied bya passenger or unoccupied. Then on the basis of all output signalssupplied by the threshold value decision circuit, a final decision wouldhave to be made about whether the occupied or unoccupied state is themost probable.

The reliability of the final decision whether or not the vehicle seat isoccupied by a passenger can be further increased by including otherexternal decision-making criteria that are independent of the sensorsignal (e.g., whether or not the seatbelt buckle is fastened).

Since instantaneous vehicle operation parameters such as engine rpm orvehicle speed also affect the occupancy sensor in the vehicle seat byinducing vibrations in the seat, it is expedient to use engine rpmand/or vehicle speed as controlled variables for some circuits in thearrangement described above. Block 8 in the FIGURE indicates engine rpmand/or vehicle speed. It is sent as a control signal to amplifier 2 offilter bank 3, selector module 4, circuit 5 for forming the mean value,the peak value or the effective value, for example, and to thresholdvalue decision circuit 7, as indicated by the dashed connecting lines.However, not all the circuit elements mentioned here need be controlledby the vehicle operation parameters. Control of amplifier 2 permitscompensation of fluctuations in the level of the sensor output signaldue to vibrations in the vehicle seat depending on the above-mentionedvehicle operation parameters. Due to the influence of vehicle operationparameters, it may be necessary to change the filter characteristics(e.g., their order, ripple or corner/cut-off frequencies) in filter bank3. It may also be necessary to make a certain selection of frequencyranges in selector module 4 as a function of the vehicle operationparameters. Vehicle operation parameters may also have an influence onwhether it is more advantageous to compare the mean values, the peakvalues, the effective values, or other values of selected signalcomponents that are suitable for the signal analysis. Therefore, thecontrol signal of block 8 is also sent to circuit block 5. Finally, theposition of the thresholds in threshold value decision circuit 7 shouldalso be optimized as a function of one or more operating parameters.

What is claimed is:
 1. An arrangement for detecting a type of occupancyof a vehicle seat, comprising:at least one sensor responding tomovements of one of a passenger and an object occupying the vehicleseat, the at least one sensor generating a sensor output signal; meansfor decomposing the sensor output signal into a plurality of signalcomponents, each of the plurality of signal components having a selectedfrequency range, the frequency range being selected as a function of atleast one vehicle operation parameter having an influence on the sensoroutput signal; means for comparing the plurality of signal components,the means for comparing generating a comparison signal; and means fordetermining whether the vehicle seat is occupied by a passenger as afunction of the comparison signal.
 2. The arrangement of claim 1,wherein the means for decomposing includes:a plurality of filters,wherein each of the plurality of filters is one of a low-pass filter, ahigh-pass filter, a band-pass filter, and a band-stop filter.
 3. Thearrangement of claim 1, wherein each of the plurality of signalcomponents has a mean value, and wherein the means for comparingcompares the mean value of each of the plurality of signal components.4. The arrangement of claim 1, wherein each of the signal components hasa peak value, and wherein the means for comparing compares the peakvalue of each of the plurality of signal components.
 5. The arrangementof claim 1, wherein each of the plurality of signal components has aneffective value, and wherein the means for comparing compares theeffective value of each of the plurality of signal components.
 6. Thearrangement of claim 1, wherein the means for comparing compares theplurality of signal components by determining a quotient of two of theplurality of signal components.
 7. The arrangement of claim 1, whereinthe means for comparing compares the plurality of signal components bydetermining a difference between two of the plurality of signalcomponents.
 8. The arrangement of claim 1, wherein the means fordetermining includes:a threshold decision circuit, the thresholddecision circuit determining whether the vehicle seat is occupied as afunction of a comparison of the comparison signal and at least onepredefined threshold value.
 9. The arrangement of claim 8, wherein theat least one predefined threshold value is a function of at least onevehicle operation parameter having an influence on the sensor outputsignal.
 10. The arrangement of claim 1, wherein the means for comparingperforms a first comparison between two of the plurality of signalcomponents, and performs additional comparisons between others of theplurality of signal components.
 11. The arrangement of claim 1, whereinthe means for comparing performs multiple simultaneous comparisonsbetween different ones of the plurality of signal components.
 12. Thearrangement of claim 1, wherein one of the at least one vehicleoperation parameter is an engine rpm.
 13. The arrangement of claim 1,wherein one of the at least one vehicle operation parameter is a vehiclespeed.